1. Field of the Invention
This invention is directed to apparatus and methods of processing image information for rendering images using halftone dot structures.
2. Brief Description of the Prior Art
In the prior art the rendering of pictorial images by printers or displays is conventionally made by building dots within halftone cells so that grey levels of an image result from halftone dots of different grey levels. Assuming, as an example, that eight pixels comprise each halftone cell it is possible to have halftone dots that vary in density from 0 to 8 in the case where binary pixels only are rendered; i.e., either a pixel is placed at a pixel location or it is not. Thus, only 9 levels of density can be represented by such a halftone cell. In an attempt to raise the number of grey levels of a halftone cell, one may increase the number of pixels encompassed by the cell, however, this has an adverse impact on image resolution.
In U.S. Pat. No. 5,198,910 to Ng et al, additional numbers of grey levels or ranges covered by grey level halftone cells is taught to be achievable by switching between different dot type growth patterns. For example, for lower densities a Bayer or dispersed type growth pattern is used wherein density of a halftone cell is increased by adding pixels at spaced locations from other pixels in the cell. Such a dot growth pattern is favored for low density regions. For higher density regions, a cluster type of growth pattern is more favored wherein growth of grey levels of the halftone cell is made by adding pixels around a central pixel of the halftone cell. A disadvantage with this approach, as well as the binary pixel halftone cell approach, is that it, too, does not provide a sufficient number of grey levels to halftone cells for providing for better quality of halftone reproductions of images. In U.S. Pat. No. 5,200,831 to H. T. Tai, there is disclosed the rendering of grey level patterns using halftone cells that are each comprised of plural grey level pixels. For example, in a two-bits per pixel system a grey level pixel may be of relative absorptance or relative spot size 0, 1, 2, or 3. With eight pixels to a halftone cell, it is possible for the cells to be of any density 0-24 and thus 25 cell density levels are realizable. While this is significantly better than the binary approach, there is a need for still higher numbers of halftone images.
In U.S. Pat. No. 4,868,587 to Loce et al and U.S. Pat. No. 5,406,379 to Kingsley et al, there is recognized that when more halftone levels are needed than can be obtained conventionally, one can introduce the concept of halftone cell density growth through the introduction of non-monotonicity in pixel growth, along with unequally spaced pixel absorptance levels. As will be described more fully below, this type of growth has corresponding pixels in halftone cells of progressively higher density halftone dots change direction in pixel growth. However, the approaches described by Loce et al and Kingsley et al require that a halftone cell can be generated by first calculating a block average, for example a 2xc3x974 block of pixels to be rendered, and then rendering the halftone cell of pixels from the average of the block. A problem with this approach is that edges passing through the block are rendered poorly since the edge information is averaged into the block. These patents recognize this problem that averaging would lose microstructure detail and suggest a need to rearrange the pixels within a halftone cell to better match the original detail. However, little guidance is offered to accomplish this result.
In multi-bit halftoning, as taught for example in the aforementioned patent to Tai, U.S. Pat. No. 5,200,831, a single threshold mask for each level efficiently renders the input image into a halftone pattern. The successive halftone patterns are correlated with the preceding pattern in the sense that pixel density for only one or a few pixels is increased from one pattern to the next, the other pixels remaining unchanged. Thus, there is no need to store in memory the complete pattern details for all the halftone levels. The masks specify the thresholds at which each pixel is to be increased to the next grey level. Image detail within the cell is largely preserved, while imposing a regular halftone dot pitch. However, the conventional threshold masks cannot deal with pixels that increase non-monotonically in density as the halftone dot is built.
For the special case of a single intermediate grey level, i.e., xe2x80x9cternaryxe2x80x9d halftoning, Moriguchi et al, U.S. Pat. No. 4,701,811 describes an implementation involving fully black xe2x80x9cfullxe2x80x9d dots and xe2x80x9chalfxe2x80x9d dots (actually 70% of the xe2x80x9cfullxe2x80x9d dot area). Thresholding is applied pixel-by-pixel without averaging, to both increase and decrease the pixel coverage. Halftone levels well beyond the number obtained conventionally are achieved, but no particular attention is paid to correlated dot building, so that the threshold switches are more numerous than necessary, which adds complexity and detracts from the halftone pattern continuity as the halftone dot is built. The connection between number of pixels per cell and optimal values for pixel grey levels is not recognized. In the particular halftone patterns shown, no attention is paid to dot stability and low granularity (see Tai, U.S. Pat. No. 5,200,831 for discussion of these issues), nor to maintaining a regular dot pitch. Furthermore, if one contemplates extending the concepts of this patent to cases of 2 or more intermediate grey levels, it is not obvious how to space the pixel grey levels for the most halftone levels.
It is, therefore, an object of the invention to provide an improved method and apparatus for generating halftone images by increasing the number of available densities to halftone dots.
In accordance with a first aspect of the invention, there is provided a method of reproducing an image with a series of patterns of halftone dots, each halftone dot being comprised of plural pixels arranged in a halftone cell, the image being derived from unrendered image data represented by pixels each having an unrendered pixel value density assignment of one of M grey levels, the method comprising generating rendered pixels by comparing for each of at least some pixels to be rendered a corresponding unrendered pixel value with threshold values associated with the corresponding position of the pixel to be rendered in a halftone cell, the rendered pixels being generated with at least R+1 levels of density including background, wherein R is a whole number greater than two and less than M; and forming halftone dots each of plural rendered pixels wherein at least some halftone dots are of a first density D1 and other halftone dots are of a second density D2 and wherein D2 greater than D1, a halftone dot of density D1 having a rendered pixel at a respective pixel location with an assigned density d1 and a halftone dot of density D2 having a rendered pixel at the same respective pixel location with an assigned density d2 and wherein d2 less than d1.
In accordance with a second aspect of the invention, there is provided a method of generating rendered image data for use in reproducing an image with a series of patterns of halftone dots, each halftone dot being comprised of plural pixels arranged in a halftone cell, the image being derived from unrendered image data represented by pixels each having an unrendered pixel value density assignment, the method comprising comparing an unrendered pixel value of a pixel to be rendered with each of a series of threshold values in accordance with respective comparison criteria; and generating a grey level for a rendered pixel of a halftone dot by having certain threshold values define increasing grey level values of the rendered pixel upon meeting respective comparison criteria and at least one threshold value of the series defining a decrease in grey level value of the rendered pixel for meeting a respective comparison criterion.
In accordance with a third aspect of the invention, there is provided an apparatus for generating rendered image data for use in reproducing an image with a series of patterns of halftone dots, each halftone dot being comprised of plural pixels arranged in a halftone cell, the image being derived from unrendered image data represented by pixels each having an unrendered pixel value density assignment, the apparatus comprising means for comparing an unrendered pixel value of a pixel to be rendered with each of a series of threshold values in accordance with respective comparison criteria and means for generating a grey level for a rendered pixel of a halftone dot by having certain threshold values define increasing grey level values of the rendered pixel upon meeting respective comparison criteria and at least one threshold value of the series defining a decrease in grey level value of the rendered pixel for meeting a respective comparison criterion.
The invention will become more apparent from the following detailed description and the appended claims when read in conjunction with the accompanying drawings.